Unconventional optical diffraction type authenticating elements are used to prove the authenticity or genuineness of documents such as bank notes, checks, and identity cards. Such optical-diffraction type authenticating elements usually contain a visually observable feature which serves to verify the genuineness or authenticity of a document. Optical diffraction type authenticating elements preferably have the following features:
1. Such an element is to show a well-defined and conspicuous optical effect under different illumination and viewing-conditions occurring in every-day use.
2. It must be capable of being mass-produced economically, but requiring a great expenditure in investment and know-how.
3. It should not be similar to any known optical-diffraction patterns.
4. It should not be capable of being manufactured from, or simulating any commercially available optical-diffraction patterns.
5. It should offer the possibility of being manufactured in numerous variants, so that it can be used in numerous variations specifically in various types of documents, currencies, and values.
6. It should have adequate durability with respect to being crumpled.
There is known from U.S. Pat. No. 4,181,700 a document which includes an authenticating element in the form of an optical diffraction grating. The authenticating element covers at least a portion of a substrate, and includes a diffraction structure in the form of a phase diffraction grating or phase hologram, which generates at least one color pattern due to diffraction of light impinging thereon. The color pattern constitutes a visually testable feature of genuineness. It is characteristic for such diffraction structures that even a small change of the viewing direction, or of the direction of illumination results in a significant change of the reflected color. For example a diffraction grating having a spatial frequency of 1000 lines/mm yields a green color when white light is incident at an angle of 30 degrees, and when the document is viewed at right angles to its plane, while a red color is produced when the light is incident at an angle of 41 degrees. In practice the document is frequently illuminated by light sources having a broad spectrum, for example one or more lamps, or sunlight, passing through a window. Hence, as a result of the superposition of several spectral regions and orders of diffraction almost any arbitrary patterns of color mixture arise. These patterns are not only difficult to define, but also change in a manner which cannot be simply described, if, for example, the direction of illumination or viewing is changed by tilting of the document. If the substrate of the document is not completely even, for example as a result of crumpling, then the diffraction structure has surface facets having different and non-definite angles of inclination. Such surface facets result in nondefinable color effects during static viewing of the diffraction structure. Hence requirements 1 through 6 are not met simultaneously.
From U.S. Pat. No. 3,412,493 there is known the employment of a diffraction structure which yields a diffraction pattern of concentric circles, or a complicated diffraction pattern of arbitrary light diffraction figures. The above-noted deficiencies arise here also.
From British Pat. No. 2,093,404 there is known an authenticating device, whose diffraction structure consists of a first diffraction grating region in the form of a circle, and of a second diffraction grating region in the form of a rectangle, and wherein the rectangle encloses the circle. A viewer sees these two regions in contrasting colors, or color spectra, in dependence of the respective illuminating conditions or viewing directions. This reference does not meet conditions 1-6 previously enumerated either.
Finally, from U.S. Pat. No. 1,996,539, there is known an embossment pattern for decorative purposes, which consists of a multiplicity of individual pattern elements, and wherein the orientation of the pattern elements along a carrier is progressively varied. The line spacing of the embossment patterns is of the order of magnitude from about 1/30 mm to about 1 mm, so that any formation of diffraction color effects is expressly avoided. Such embossment patterns are therefore not suitable either to be employed as authenticating elements in documents, since they do not meet requirements 1-6.